Recently, much excitement has been generated by strong suggestions that stem cells isolated from diverse somatic tissues may have a previously unsuspected degree of development plasticity. For example, a hematopoietic stem cell may be capable of producing mature liver cells, muscle tissue, or even neurons. Similarly, central nervous system stem cells or muscle stem cells may be capable of producing mature blood cell populations. These observations have called into question several fundamental dogmas of developmental biology. In addition, these observations offer extraordinary promise in the clinical setting. It is of paramount importance to rigorously assess the suggested plasticity phenomena using precise clonal analysis. Such analysis is a common thread through several decades of studies that have defined the hematopoietic stem cell, and its functional properties. These criteria must be applied to all stem cell studies. The proposed studies encompass three Specific Aims, and will address stem cell plasticity at the organismal, the cellular, and the molecular levels. We propose clonal studies to precisely measure the plasticity of the hematopoietic stem cell in terms of its ability to contribute to non-hematopoietic tissues. In order to explore the plasticity phenomena in more direct ways, it is necessary to develop in vitro systems where such behavior can be recapitulated in a well-defined setting. We propose to develop cell culture systems to explore the plasticity of hematopoietic stem cells. Finally, stem cell plasticity will be governed, at least in part, by cell-autonomous mechanisms; that is, those mediated by the panel of gene-products expressed in stem cells. We will extend our ongoing hematopoietic stem cell gene expression analyses to stem cell populations obtained from other tissues. Molecular and computational bioinformatic comparisons should provide gene expression profiles common to different sources of stem cells, as well as those that may define tissue specific gene expression differences. Collectively, the anticipated results will provide a clonal and precise assessment of stem cell plasticity. In addition these studies will offer novel insights, as well as cellular and molecular tools to pursue stem cell plasticity in a direct mechanistic manner.